This invention relates to a cathode ray tube (CRT) for use as a color reference, and more particularly relates to such a tube in which the reference color is produced by the combined output of individual phosphor elements having different component colors. The invention also relates to a method for producing such a tube.
In U.S. Pat. No. 4,607,188, a color reference CRT is described in which the reference color is produced by the combined output of individual phosphor elements having different component colors, e.g., interlaced fields of the component colors formed by a pattern of repeating vertical stripes of red, green and blue emitting phosphors.
The tube is similar in construction to the standard color CRT used in color TV, except that it lacks a color selection electrode, and in operation the screen is scanned with one or more electron beams of fixed voltage and current, so that the output is observed as a single, invariant color, which is the result of the eye integrating the separate luminous outputs of the interlaced fields of the component colors.
In such a tube, a color reference having a desired color temperature is obtained by the appropriate selection of the component colors and the control of their luminous outputs by adjusting the relative sizes of the individual phosphor elements of the component color fields. As described in the patent, the latter adjustment was achieved by varying the exposure dosages (combination of time and intensity) used in the standard photolithographic process to produce the component color fields for color TV tubes.
While a main advantage of this method is that it can be carried out on a standard manufacturing line for color TV tubes using the standard color selection electrode as the exposure mask, an attendant drawback is that the size of the apertures in the color selection electrode varies from center to edge, and the responses of the component color fields to the exposures varies with both the aperture size and the component color.
Consequently, it has been observed that the color varies from center to edge of the screen, and that consequently only about a 4 inch square area in the center of the screen is actually useable as the color reference.